1. BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an aromatic copolyester composition having improved crazing resistance and thermal stability, and more specifically, to an aromatic copolyester composition containing a metal salt of an organic carboxylic acid and having improved crazing resistance and thermal stability.
2. Description of the Prior Art
Aromatic copolyesters, as used in this invention are obtained from a mixture of terephthalic acid and/or functional derivatives thereof and isophthalic acid and/or functional derivatives thereof (with the terephthalic acid unit/isophthalic acid unit mole ratio being about 9:1 to about 1:9) and a bisphenol of the following general formula (I): ##STR1## wherein --X-- is selected from the group consisting of --O--, --S--, --SO.sub.2 --, --SO--, --CO--, an alkylene group containing 1 to 4 carbon atoms and an alkylidene group containing 1 to 4 carbon atoms, and R.sub.1, R.sub.2, R.sub.3, R.sub.4, R'.sub.1, R'.sub.2, R'.sub.3 and R'.sub.4, which may be the same or different, each represents a member selected from the group consisting of a hydrogen atom, a chlorine atom, a bromine atom and an alkyl group containing 1 to 4 carbon atoms, or functional derivatives thereof.
It is well known that such aromatic copolyesters have many superior properties, for example, superior mechanical properties such as tensile strength, bending strength, bending recovery, or impact strength, a high heat distortion temperature, and a high heat decomposition temperature, electrical properties such as inherent resistivity, dielectric breakdown strength, arc resistance or dielectric characteristics, good fire retardancy, good dimensional stability and good solvent resistance. Because of these superior properties, aromatic copolyesters are known to be useful in a wide range of fields as various molded articles, films, fibers and coating materials produced by injection molding, extrusion molding, press molding, and other molding techniques.
Aromatic copolyesters have excellent characteristics and as a result can be used in a wide variety of applications as mentioned above, but unfortunately, crazing develops in products produced from these aromatic copolyesters, especially molded plastics, when they are in contact with air for a long period of time or especially when in contact with hot water or steam. The term "crazing" is used herein to describe the phenomenon whereby molded products become cloudy, partly or completely, when a fine crack-like pattern being generated on the surface or in the interior of the products. "Crazing" reduces not only the transparency of the products, which is one of the characteristics of aromatic copolyesters, but also decreases their toughness. Thus, for example, their tensile impact strength is markedly reduced, and their bending strength, modulus of bending and recovery ratio after bending are also reduced. As a result, when the products are bent, breaking tends to occur.
Some methods for preventing crazing of aromatic copolyesters are known. For example, a method involving mixing and melting polyethylene terephthalate with the aromatic copolyester (e.g., as described in U.S. Pat. No. 3,946,091), a method comprising mixing and melting polyethylene hydroxybenzoate with the aromatic copolyester (e.g., as described in U.S. Pat. No. 3,884,990), a method involving mixing and melting polybutylene terephthalate with the aromatic copolyester (e.g., as described in Japanese Patent Application (OPI) No. 34342/75), and a method comprising mixing and melting an organic phosphite ester with the aromatic copolyester (e.g., as described in Japanese Patent Application (OPI) No. 16558/72), are known attempts to prevent crazing. All of these methods, however, reduce the inherent fire retardancy of aromatic copolyesters, and also tend to decrease the high heat distortion temperatures and thermal stability of aromatic copolyesters. Other improvements have therefore been desired.
Frequently, aromatic copolyesters are used at high temperatures to take advantage of their high heat distortion temperatures. However, the heat stability of aromatic copolyesters at high temperatures is not entirely satisfactory, and an improvement in the heat stability has been desired.
Melt-mixing of a phosphite ester with aromatic copolyesters (e.g., as described in Japanese Patent Application (OPI) No. 16558/72), and the addition of an epoxy compound to aromatic copolyesters are known examples of methods for improving the thermal stability of aromatic copolyesters. However, these methods cause a reduction in the fire retardancy of the copolyesters, and better methods are desired.